Such networks are constituted by a plurality of nodes interconnected by optical links. Each node is provided with "user" inlets and is connected to one or more other nodes by means of corresponding optical lines.
A user message is conveyed from a transmitter node to a destination mode via an optical link comprising lines and possibly intermediate nodes, along a route that is generally a function of the wavelength carrying the message. This spectrum routing function is controlled by a network manager which allocates the wavelengths carrying the messages at each transmitter node and as a function of the destination modes.
Unfortunately, the performance of such a network, in particular in terms of signal quality and of data rate is limited by the optical properties of the links which give rise to physical phenomena causing the optical signals to be degraded. Such phenomena are in particular chromatic dispersion and nonlinearity in the fibers making up the lines, as well as attenuation of the optical power in the fibers, and in certain components such as couplers, switches, multiplexers, demultiplexers, and wavelength converters used in the nodes and all along the lines.
However, means are known for remedying, at least in part, the degradation that such phenomena cause. Thus, attenuation in the fibers can be compensated by means of amplifiers disposed upstream or downstream or all along links. Another known technique referred to as "spectrum inversion" or "phase conjugation" can be advantageously used to correct the effects of chromatic dispersion as well as nonlinearities due to the Kerr effect (variations in the index of a medium depend in linear manner on variations in light intensity). The principle of compensation using spectrum inversion is described in the article "Compensation for channel dispersion by nonlinear optical phase conjugation", Amnon Yariv et al., Optics Letters, Vol. 4, No. 2, February 1979. The conditions to be satisfied for spectrum inversion to correct both chromatic dispersion and nonlinearities were described by S. Watanabe at the IOOC 95 conference that was held from Jun. 26 to 30, 1995 in Hong Kong.
For reasons of simplicity, attempts to improve the performance of a network generally involve compensating or pre-compensating the above-mentioned phenomena at the ends of the lines. Unfortunately, that type of correction, referred to as "point-to-point compensation" assumes that the ends of the fibers are accessible so that it is possible to interpose compensation apparatuses between the ends and the nodes. Such modification is costly, and is not always possible in already-installed networks, especially when the networks are large.